theorem Th70:
  proj2.:LSeg(SW-corner C,NW-corner C) = [.S-bound C,N-bound C.]
proof
A1: (NW-corner C)`2 = N-bound C by EUCLID:52;
  (SW-corner C)`2 = S-bound C by EUCLID:52;
  hence thesis by A1,Th22,Th53;
end;
